1. Field of the Invention
This invention relates generally to television aperture masks and, more specifically, to a process for forming openings of substantially the same sizes in a television aperture mask.
2. Description of the Prior Art
The concept of aperture masks for television picture tubes is well known in the art. A typical prior art aperture mask is shown in the Braham U.S. Pat. No. 2,750,524 which shows an aperture mask having a plurality of circular openings.
The operation of such aperture masks in a television picture tube may be found in the Fyler, et al. U.S. Pat. No. 2,690,518 which shows a color television tube having an aperture mask located as an electron beam screen.
The prior art aperture mask openings have taken many different shapes including round as shown in the aforementioned patents or elongated as shown in the Suzuki, et al. U.S. Pat. No. 3,882,347. While the shape of the opening may vary in different masks, generally all masks require the open area in the aperture mask to be graded to to accommodate the characteristics of the human eye. That is, if a television picture is to appear uniform in brightness to the human eye, it is necessary to have a televison picture where the central area of the television picture is actually brighter than the peripheral area. To obtain a brighter central area the aperture masks are usually made with larger size openings in the center of the mask and smaller size openings in the periphery of the mask with openings of intermediate sizes located therebetween. As the brightness of a television picture tube is directly proportional to the open area of the aperture mask, the use of a constant density of apertures with gradually decreasing size produces an image that appears uniform in brightness to the human eye. Typically, the brightness at the center of the aperture mask, is greater than at the peripheral region of the aperture mask.
The prior art Tsuneta, et al. U.S. Pat. No. 3,652,895 shows an aperture mask having a plurality of rectangular slots or circular openings with the size of the openings decreasing in size from the center of the mask to the peripheral portion of the mask. FIG. 13 of the Tsuneta, et al. patent also shows an alternate concept in which instead of varying the aperture size, the space between apertures is increased to thereby decrease the open area on the peripheral regions of the mask. While the concept of decreasing the density of apertures in the periphery of the aperture mask is well known, the method of making an aperture mask with substantially the same size openings and within proper tolerances has been quite difficult. The prior art Tsuneta U.S. patent states that he obtains rectangular slots by etching most part of the thickness of the mask plate from the upper side and then etching the remaining part from the lower side of the plate.
Still another method of decreasing the size of the openings in an aperture mask is taught in the Frantzen, et al. U.S. Pat. No. 3,788,912, Frantzen, et al. teaches the nozzle position and the amount of spray can be varied to provide larger or smaller openings in selected regions of the mask. In the Frantzen technique the openings in the photoresist are of equal dimensions throughout the aperture mask with control of the aperture size obtained through controlling the etchant supply.
Typical aperture masks in use today are made from a base material and have a cone side surface and a grade side surface. The cone side surface comprises a set of hollowed out recess regions located on one side of the aperture mask. Located in the hollowed out recess region is an elongated or circular aperture. The side opposite the cone side surface is known as the grade side surface.
To enlarge the apertures one may vary the spray time in mass production lines or adjust a series of multiple etching stations such as shown in Frantzen U.S. Pat. No. 3,788,912. However, such techniques are difficult to use and depend a great deal on the skill of the operator.
The invention shown in my U.S. Pat. No. 4,303,466 teaches the sizing of etchant resist openings located on the cone side of an aperture mask by varying the cone size openings in the etchant resist while maintaining constant size grade side openings yet maintaining a substantial constant over-etch factor throughout the aperture mask even though the size of the cone side etchant resist openings and the openings in the aperture mask vary throughout the aperture mask.